1. Field of the Invention
The present invention relates to a transmission circuit that is utilized in a communication device such as a mobile phone, a wireless LAN, and the like. More specifically, the present invention relates to a transmission circuit that conducts bias control of a power amplifier.
2. Description of the Background Art
In recent years, in a highly information-oriented society, a communication device such as a mobile phone, a wireless LAN, and the like, have to ensure a linear transmission signal in a broad power amplification range while achieving low power consumption. Adopted in such communication device is a transmission circuit which outputs a transmission signal with high precision regardless of the operation bandwidth and which operates with high efficiency. A conventional transmission circuit is described in the following.
As a conventional transmission circuit, for example, there has been a transmission circuit that generates a transmission signal by utilizing a modulation method such as orthogonal modulation and the like (hereinafter, referred to as an orthogonal modulation circuit). The orthogonal modulation circuit is widely known, therefore description thereof is omitted. Furthermore, a transmission circuit 50 shown in FIG. 10 is an example of a conventional transmission circuit that operates with higher precision and higher efficiency than the orthogonal modulation circuit. FIG. 10 is a block diagram showing one configuration example of the conventional transmission circuit 50. In FIG. 10, the conventional transmission circuit 50 includes: a signal generator 501; a phase modulator 502; a regulator 503; a power amplifier 504; and a power supply terminal 505. The power amplifier 504 includes a transistor for amplification.
In the conventional transmission circuit 50, the signal generator 501 generates an amplitude signal and a phase signal. The amplitude signal is inputted into the regulator 503. Furthermore, a DC voltage is supplied from the power supply terminal 505 to the regulator 503. The regulator 503 supplies a voltage to the power amplifier 504 depending on the amplitude signal inputted into the regulator 503. Typically, the regulator 503 supplies the power amplifier 504 with a voltage that is proportional to the level of the inputted amplitude signal.
On the other hand, the phase signal is inputted into the phase modulator 502. The phase modulator 502 conducts a phase modulation on the phase signal, and outputs a phase modulation signal. The phase modulation signal is inputted into the power amplifier 504. The power amplifier 504 conducts an amplitude modulation on the phase modulation signal by using the voltage supplied by the regulator 503, and outputs the resulting signal as a modulation signal that has been phase modulated and amplitude modulated. This modulation signal is outputted from an output terminal as a transmission signal. Such transmission circuit 50 is called a polar modulation circuit.
Although the transmission circuit 50 described above can operate with high precision and high efficiency, the transmission circuit 50 requires further reduction in power consumption if being used as a mobile wireless device. In particular, in the transmission circuit 50 described above, since a large part of the power is consumed by the power amplifier 504 which is a part that conducts power amplification of an output, reduction in power consumption of the power amplifier 504 is required.
One example of an approach to reduce power consumption of a power amplifier, not limited to a polar modulation circuit, is a bias control technology disclosed in Japanese Laid-Open Patent Publication No. H5-110348 (hereinafter, referred to as patent document 1). Patent document 1 discloses a high-frequency wave amplifier that switches a bias voltage by using a bias-switching switch in accordance with an operation condition of the power amplifier. In general, since amplification of an analog FM modulated wave only requires transmission of a phase change, a usage of a class-C power amplifier that has high power conversion efficiency is preferred. On the other hand, in the case of a modulation method for a digitally modulated wave, for example a π/4 shifted QPSK modulated wave, transmission of both amplitude and phase change is required, thus, a usage of a class-A power amplifier having a fine linearity is necessary. Patent document 1 discloses a configuration that allows control of an operation range of the power amplifier by switching the bias voltage applied on a transistor for amplification, depending on the modulation signal.
FIG. 11 is a block diagram showing one configuration example of a transmission circuit 51 that uses a conventional high-frequency wave amplifier. In FIG. 11, a modulated wave inputted from a RF input terminal 511 is amplified by a power amplifier 512, and outputted from an antenna 513. The power amplifier 512 includes: a transistor for amplification 518 that is emitter grounded; an input side matching section 517 connected to a base of the transistor for amplification 518; and an output side matching section connected to a collector of the transistor for amplification 518.
A switching control circuit 515 is connected, through a bias-switching switch 520, to the base of the transistor for amplification 518 which is a component of the power amplifier 512. The bias-switching switch 520 is connected to the base of the transistor for amplification 518 on either a first bias circuit 522 side or a second bias circuit 523 side, depending on a control signal which is for modulated wave selection and which is inputted from a control signal input terminal 514. More specifically, the bias-switching switch 520 is connected to the base of the transistor for amplification 518 on the first bias circuit 522 side when the power amplifier 512 functions as a class-C power amplifier. On the other hand, the bias-switching switch 520 is connected to the base of the transistor for amplification 518 on the second bias circuit 523 side when the power amplifier 512 operates as a class-A power amplifier. As described above, the conventional transmission circuit 51 converts the operation range of the power amplifier 512 by switching the bias circuit depending on the type of the modulated wave of which the power amplifier 512 amplifies, thus enabling improvement in power conversion efficiency of the conventional transmission circuit 51.
However, in the conventional transmission circuit 51, when switching between the first bias circuit 522 and the second bias circuit 523, a state in which a bias is not applied (non-bias) to the power amplifier 512 can be generated, because of a delay in switching operation during a bias circuit switching, or because of a delaying element that exists in a control signal pathway.
The switching operation during the bias circuit switching is described with reference to FIG. 12. FIG. 12(a) shows a change in a bias voltage when a control signal is inputted into the control signal input terminal 514 at time t0 and the first bias circuit 522 is switched to the second bias circuit 523. In FIG. 12(a), when comparing before time t0 and after time t0, the bias voltage changes from V1 to V2 in a step-like way due to the control signal inputted into the control signal input terminal 514.
FIG. 12(b) shows one example of a delay that is generated in a bias current which is applied to a bias control terminal of the transistor for amplification 518. As shown in FIG. 12(b), a delay in the switching operation of the bias current which is applied to the bias control terminal of the transistor for amplification 518 is generated, due to influences of a delay characteristic of the bias-switching switch 520 and a delaying element that exists in a transmission pathway of a communication information signal. In this example, delay time from the time t0 when the control signal is inputted into the control signal input terminal 514 up to the time when the first bias circuit 522 is switched from on to off is defined as d1, and up to the time when the second bias circuit 523 is switched from off to on is defined as d2 (d2>d1). In this case, during a time period of T1=d2−d1, both the first bias circuit 522 and the second bias circuit 523 are off, and the transistor for amplification 518 is in a non-bias state.
Therefore, the above described transmission circuit 51 has a problem of not being able to conduct a smooth switching operation of the power amplifier 512, due to an interruption of the output signal of the power amplifier 512 during bias current switching, since the transistor for amplification 518 does not conduct a normal operation in a non-bias state.
Furthermore, even if the bias control technology disclosed in patent document 1 is applied to the transmission circuit 50 (polar modulation circuit) described above, the transistor for amplification that includes the power amplifier 504 cannot be prevented from entering a non-bias state.